Polymers- Blow Moulding, Rotational Moulding, Calendering

Question 1

What is blow moulding used for?

Answer 1

Hollow parts (bottles, containers). Higher melt strength thermoplastics like PE and PET. Relatively thin wall thickness. Relatively little scrap

Question 2

How does extrusion blow moulding work?

Answer 2

Extruder a molten tube (parison) through a die. Close the mould on it. Insert a blowing pin in the bottom opening. Inflate the parison to fill the mould. Cool, eject, repeat

Question 3

Features of EBM

Answer 3

Relatively low levels of polymer orientation (some from extrusion and inflation). Can vary parison thickness to ensure constant wall thickness (by raising or lowering die mandrel). Where mould closes creates pinch off (scrap and weak point on final product). Can make containers large and small (up to 200L)

Question 4

How to improve efficiency of EBM

Answer 4

Extrusion phase is quicker than inflation, cooling, ejection. Use more than on extruder and a multiple cavity mould. Can extruder into moulds on a rotating wheel

Question 5

Problem with EBM for large moulds and solution

Answer 5

Parison can sag under its own weight and tear. Use an accumulator (hot chamber) to collect enough polymer melt to perform the extrusion as quickly as possible

Question 6

How does injection blow moulding work?

Answer 6

Injection mould a preform (correct mass but different shape). Soften the preform and load into the blow mould. Insert blowing pin and inflate into shape of mould. Cool, remove blowing pin and open mould to remove part.

Question 7

Features of IBM

Answer 7

Medium levels of polymer orientation. Injection moulding a high pressure process, some orientation during inflation. Higher stiffness, lower permeability, better clarity than EBM. No pinch off. Requires two moulds. Used for transparent bottles with threaded necks.

Question 8

Stretch blow moulding

Answer 8

Similar to IBM but softened preform is mechanically stretched by the blowing tube before inflation. High levels of polymer orientation. Biaxial orientation from stretching and inflation. Highest stiffness, lowest permeability, best clarity. Allows use of higher molecular weight polymers. Used for drink bottles (all PET carbonated drinks and water)

Question 9

Design of blow moulding moulds

Answer 9

Control wall thickness. Rounded corners to allow complete mould filling and prevent weak areas. Thickened bases prevent pinch-off weakness. Mould filling displaces air which could overheat and scorch part but vents, holes or sintered plugs allow air escape.

Question 10

Why blow mould and not injection mould?

Answer 10

For bottles is cheaper. Requires medium pressures so cheaper tooling and mould materials such as Al or Zn alloys can be used

Question 11

How does general rotational moulding work?

Answer 11

Plastic is heated and flows onto the inside or a rotating mould. Charge the mould. Gravity (not centrifugal force) causes the charge to coat the inside of the mould. Cool. De-mould (open mould)

Question 12

Features of rotational moulding

Answer 12

Low pressure alternative for hollow part production (1atm). Little to no polymer orientation induced by the processing.

Question 13

What is rotational moulding used for?

Answer 13

Thin or thick-walled parts. Large, seamless parts with or without openings. Road furniture (bollards, cones, barriers), storage tanks, bins, buoys, toys.
Traditionally used with LVC in plastisol form (suspension of a fine polymer powder in a liquid plasticiser)

Question 14

Stations of rotational moulding

Answer 14

There are 3 and move to one at a time. Plastic addition. Then heating station while being rotated in all directions. Then cooling station while being rotated in all directions. Back to part removal (first one) station where mould is opened to leave moulded part

Question 15

Multilayer moulding (rotational)

Answer 15

Do rotational moulding for outer material first, then do again with inner material inside it. Need to consider chemical compatibility (adhesion), thermal compatibility, shrinkage.

Question 16

Foamed interior with rotational

Answer 16

Structural integrity from solid outer layer but low weight from foamed inner layer. Can use different sized particles. Smaller ones sinter to mould first to create solid outer layer, larger ones sinter to outer layer and contain the blowing agent

Question 17

How does calendering work?

Answer 17

Force a softened polymer rolling bank through a series of gaps (nips) between counter-rotating rollers to produce sheet or film. High pressures build up on the rollers. Final thickness dependent on last roll gap

Question 18

Features of calendering

Answer 18

Temperatures and shear forces relatively low. Process can be primary (manufacture of sheet from raw materials) or secondary (add gloss or embossing to existing sheet. Expensive equipment. Initial shearing aligns polymers to some extent but overall orientation limited

Question 19

Positives of calendering over sheet/film extrusion

Answer 19

Provides better surface finish, thickness tolerance and production rate.

Question 20

What is calendering used for?

Answer 20

PVC (because of lower temperatures so no HCl production) and elastomer sheets. Flooring tiles and coverings. Cladding/sheeting. Waterproof membranes, tank liners, conveyor belts

Question 21

Roll configurations for calendering

Answer 21

I type is original and is rollers in straight line (roll separation forces). L type is L shape of rollers (roll separation forces). Z type is best because reduced roll separation and reduced heat loss as limited contact with each roller.

Question 22

What happens if sheet from calendering is too thick or thin?

Answer 22

Too thin: pinholes and pores

Too thick: trapped air pockets